It'll be very interesting when Imperial Guard, Orks, and Tyranids get their new codex, considering in 5th edition, these were relatively "spam heavy" in models, particularly in Troops.
That being said, I can comment a bit further on Space Marines and Eldar, since I've played both regularly since their new codices were released.
Space Marines: Tactical Squads are still as versatile as they were, and a lot more point efficient. Having the ability to choose between a Heavy or Special Weapon when you have a squad between 5 and 9 models is nice too. Scouts got better, but more as a result of the new 6th Edition rules (ie - Precision Shooting, Sniper Weapons), than with the new codex, although they are cheaper now as well. Sgt. Telion is worth every bang for his buck.
Eldar: Guardians got a lot better. And I mean, A LOT better. No longer are they "overpriced Guardsmen". Their heavy weapon platforms becoming Relentless makes them more reliable, and now that Scatter Lasers function like "co-axial" weapons, they're very dangerous in upclose shooting (Possibility of 40 Twin Linked Shuriken Catapult Shots on Overwatch is ridiculous) That being said though, Dire Avengers are still the better choice over Guardians, but not by far. With the Counter Attack special rule, they're a bit more reliable in Close Combat, but only for a turn. The Exarch's Shimmershield now works ALL the time, as opposed to only when the unit was in Close Combat. Lastly, with the changes to how Bladestorm works, BOTH units are a threat to kill units with 2+ Armor Saves.
